This invention relates to an extramedullary angle indicator, i.e. goniometer for measuring the version angle of the humeral head. The version angle is then used to orient a humeral prosthesis in the canal of the humerus.
One of the most critical steps in successful shoulder replacement is determining and maintaining the version of the humeral head. Most surgical techniques for total shoulder replacement advise the surgeon that the humeral component should be set at approximately 30-40° of retroversion. This will restore the shoulder's range of motion and the overall stability of the joint. Although the recommendations for humeral head retroversion are similar in most shoulder surgeries, the method in which this version is measured varies.
A prior art technique developed by Stryker Corp. for their Solar™ total shoulder, as shown in FIG. 1, instructs the surgeon to flex the patient's elbow 90° and externally rotate the forearm 30-40°.
In this system, as shown in FIG. 1, the surgeon uses the patient's sagittal plane (90° arrow) as a zero reference while rotating the forearm 30-40° away from the body. At this point, the surgeon visually sets the forearm at 30-40° and thereafter makes the humeral head resection in a known manner (not shown). The humeral head resection cut is made at right angles to the plane of the humeral shaft and head, thereby achieving 30-40° of retroversion.
As used herein, when referring to bones or other parts of the body, the term “proximal” means closer to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means towards the head. The term “anterior” means towards the front part of the body or the face and the term “posterior” means towards the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. The sagittal plane is a vertical plane running from head to toe and dividing the body into right and left sections. The frontal or coronal plane is a vertical plane dividing the body into front and back sections.